The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating, but when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes at least one coil, a write pole and one or more return poles. When a current flows through the coil, a resulting magnetic field causes a magnetic flux to flow through the write pole, which results in a magnetic write field emitting from the tip of the write pole. This magnetic field is sufficiently strong that it locally magnetizes a portion of the adjacent magnetic disk, thereby recording a bit of data. The write field, then, travels through a magnetically soft under-layer of the magnetic medium to return to the return pole of the write head.
A magnetoresistive sensor such as a Tunnel Junction Magnetoresisive (TMR) sensor can be employed to read a magnetic signal from the magnetic media. The sensor includes a non-magnetic, electrically insulating barrier layer (if the sensor is a TMR sensor) sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. Magnetic shields are positioned above and below the sensor stack and can also serve as first and second electrical leads so that the electrical current travels perpendicularly to the plane of the free layer, tunnel barrier layer and pinned layer (current perpendicular to the plane (CPP) mode of operation). The magnetization direction of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetization direction of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The electrical resistance through the barrier layer (insulator material) of a magnetic tunneling junction changes with the relative orientation of the free layer magnetization with respect to the pinned layer magnetization. When the magnetizations of the pinned and free layer are parallel with one another, a lower resistance is detected and when the magnetizations of the pinned and free layer are antiparallel a higher resistance is detected. In a read mode the resistance of the TMR sensor changes about linearly with the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the TMR sensor, resistance changes cause potential changes that are detected and processed as playback signals. In the case where the barrier layer is comprises MgO, if the MgO has a crystalline (100) texture, adjacent to the pinned layer and the free layer has a BCC crystal structure with (100) texture, implying coherent tunneling and high TMR ratio, the TMR ratio is maximized.
With the push for ever increased data density, it become necessary to constantly improve sensor performance and robustness. Therefore, there remains a need for an improved sensor that having improved pinning strength, improved free layer stability and improved sensitivity.